Ongoing discussions of turn signal flasher problems and dim tail lights brought me to post this page about trailer tail lights. The first thing you may notice when you plug in the trailer lights is that the turn signals on the car may not blink at the same rate. This is because the turn signal flasher unit is load sensitive, designed to work with the electrical load of exactly two bright incandescent bulbs connected to it (2 x 21 watts = 42 watts electrical load). If one bulb should burn out or become disconnected, the reduction of electrical load will make the flasher unit stop flashing, while the remaining good bulb stays lit. This gives the driver immediate notice that the turn signals are not working properly and need to be fixed.

When you plug in the trailer you are adding a third turn signal bulb to this circuit. Various turn signal flasher units may react differently depending on internal design. The one on my MGA slows down considerably, staying on for a longer time with each flash cycle, with off time about the same as original. This slower flash rate may be less noticeable to other drivers around you, so it is desirable to do something to restore the original flashing rate.

The proper fix would be to install a load sensitive turn signal flasher unit designed to operate three bulbs, so if one bulb burns out it stops flashing. This works fine as long as the trailer is plugged in during a long road trip, but when you disconnect the trailer you have to switch back to a 2-bulb flasher unit or the car turn signals will not flash at all. This is extremely inconvenient if you disconnect the trailer for a few hours or a day during the long road trip (a very common occurrence).

The next best fix, and much more convenient, is to install a heavy duty flasher unit as is used for 4-way hazard flashers. This flasher is designed to flash at a fixed rate regardless of the number of bulbs connected, usually allowing for any number from 1 to 6 bulbs (sometimes more). Reason behind this design is in case of an accident where one lamp fixture may be smashed and inoperative, the flasher unit will still make the remaining hazard lights flash properly. This works great with the trailer plugged in, but you need to keep in mind that you give up the convenience and safety feature of notification of an inoperative bulb. As such, as long as you use a heavy duty flasher unit in place of the turn signal flasher unit you need to make frequent external inspection of the turn signal lamps to be sure they are always operating as expected.

If you tow a trailer often and switch between trailer and no trailer frequently (as I do), you might consider installing two turn signal flasher units (a 3-bulb flasher along with the original 2-bulb flasher). You can mount them side by side, wired in parallel, with a double throw toggle switch to change input power between the two units. Then when you connect or disconnect the trailer you can just throw the switch, and you will always have the proper safety type flasher unit that will stop flashing with a non-functional lamp. For a car that has a plug-in socket for the turn signal flasher unit you can switch flasher units (rather than installing a switch) and keep the alternate one in the car. This may depend on how easy (or difficult) it is to access the flasher unit socket.

On another issue, a trailer will often have an inordinate number of number of electrical connections along the wiring harness, and these may not be of the best design for good long term service. Some trailer wiring is put together using those dreaded pinch-on connectors that tend to fail or corrode and go to high resistance after a while. A trailer hitch electrical connector on the tow vehicle is also commonly installed with the same crummy pinch-on connectors. These usually work okay for a while after installation for temporary use, but I don't think they were ever intended to be used for reliable long term service.

A trailer may also present a number of problems with return ground connections with lights commonly being grounded through the trailer chassis, and the related problems of deteriorating ground connections. At one time my trailer got so bad that the tail lights were extremely dim, the brake lights were similarly not particularly bright, and one tail light might go out when the brakes were applied. These are all classical symptoms of high resistance wiring connections. If you consider the total electrical path from one battery post to the other post by way of trailer light fixtures, you may find more than 50 feet of small gauge wire for each turn signal and tail lights, and dozens of possible problematic wire connections along the way.

If you have dim bulb problems you will need to figure out where the electrical voltage is being lost. The best first move is to check voltage at the connector between the tow vehicle and trailer while the lights are on. Have someone step on the brake while you check voltage on the brake light wire as well as the tail light wire. For a 12-volt electrical system you had better have at least 11-volts here under load, or the trailer lights will be starting with a severe handicap. Fix any vehicle wiring problems before you try to tackle dim trailer light issues.

For effective trailer light debugging it is best to begin with by passing the tow vehicle wiring all together. Use a separate fully charged battery to connect the trailer lights, or run jumper wires directly from the vehicle battery to the trailer light connector. Again check voltage at the trailer connector to be sure you have adequate input supply voltage before going any farther. Then use a test light or volt meter with a long jumper wire connecting to the ground return terminal on the trailer connector. First probe the input power terminal to determine good connections and/or known supply voltage. Then probe connecting points along the wiring path in the trailer between the power input connector and the lamps, and compare observed voltage to the input voltage to track down bad connectors or broken wires. When you go past a bad connector there will be a noticeable drop of voltage. Stop right there and fix the connector.

As you go past the lamp fixture there will be a large drop of voltage (hopefully down to near zero) due to internal resistance of the light bulb. Debugging the ground return side is similar except connect one lead of the volt meter to the power input terminal of the supply connector. Then start probing at the ground terminal of the supply connector (full voltage) and work your way along the grounding path from the supply connector toward the lamp fixture. Bad connections will show as a noticeable change of voltage from one connection point the next.

When you think you have all of the supply and grounding connections working properly, the final test is to check voltage between supply and ground return terminals of the lamp fixture itself (with lights on under load). If you don't have at least 10 volts at every lamp fixture (when plugged into the tow vehicle) you still have a problem, and you need to keep working on the connections for supply and ground wires. If/when you get frustrated enough you may go on to removing wire connectors, soldering wires together, and installing shrink tubing to cover and insulate the wiring connections. The more connectors and bad connections you can replace with solder joints the fewer problems you will have with trailer wiring in the future.

I finally came to a better solution for all of these problems. In 2008 I bought a pair of LED trailer lights from Harbor Freight for $10 each. They work great as direct replacement for standard trailer lights where you have the space to mount them. Four LEDs in the center have narrow cone projection and appear very bright to the rear but not so much from the side. Six LEDs in a circle around the outside have wide cone projection and can be seen well as much as 60-degrees off to the side. Notice the center LEDs appear brighter in the picture, even though this is just tail lights, not brake lights. This use of both narrow and wide angle projection in the same unit has good merit.

The day is coming when brighter LEDs or higher density clusters will be available for automotive applications. Meanwhile, use of LED lamps for the trailer has a couple of very significant advantages. First, the LEDs draw very little electrical power or current, so it does not affect the turn signal flasher unit or flashing rate when the trailer is connected. Second, with very low current in the trailer wiring circuits the voltage drop is almost nil, so there will be maximum voltage available to operate the trailer tail lights and turn signals. Additionally, with less total current and less voltage drop the side marker lights and license plate lights on the trailer will also be brighter.